Air bearing assembly for guiding motion of optical components of a laser processing system

ABSTRACT

A rigid support structure allows for faster and more accurate positioning of axially adjustable optical components in a laser processing system. Vibrational and thermal stability is improved when an optics assembly is housed in a rigid air bearing sleeve that is mounted to a support structure above a specimen stage.

COPYRIGHT NOTICE

©2007 Electro Scientific Industries, Inc. A portion of the disclosure ofthis patent document contains material that is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent document or the patent disclosure,as it appears in the Patent and Trademark Office patent file or records,but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d).

TECHNICAL FIELD

The present disclosure relates to specimen processing systems and, inparticular, to stage architecture for control of two- orthree-dimensional positioning of a processing device relative to atarget specimen.

BACKGROUND INFORMATION

Wafer transport systems configured for use in semiconductor wafer-levelprocessing typically include a stage having a chuck that secures thewafer for processing. Sometimes the stage is stationary, and sometimesit is moveable. Some applications require that the stage move linearlyin one, two, or three Cartesian dimensions, with or without rotation.The speed of the stage motion can dictate the throughput of the entirewafer processing platform if a significant amount of the total processtime is spent aligning and transporting the wafer.

For applications including optical processing, a moveable opticsassembly can be mounted above the wafer surface, thereby minimizing thewafer transport distances required. The primary direction of stagemotion is referred to as the “major axis,” and the direction of stagemotion perpendicular to the primary direction is referred to as the“minor axis.” The chuck holding the wafer, or specimen, to be processedmay be mounted to a major axis stage for movement along the major axis,a minor axis stage for movement along the minor axis, or in stationaryposition below the major and minor axes. The major axis stage maysupport the minor axis stage, or they may be independent of each other.

Stage design of such optical systems is becoming more critical aselectrical circuit dimensions shrink. One stage design consideration isthe impact of process quality stemming from vibrational and thermalstability of the wafer chuck and optics assembly. In the case in whichthe laser beam position is continually adjusted, state-of-the-artstructures supporting the laser assembly are too flexible to maintainthe required level of precision. Moreover, as circuit dimensions shrink,particle contamination becomes of greater concern.

SUMMARY OF THE DISCLOSURE

A “split axis stage” architecture is implemented as a multiple stagepositioning system that, in a preferred embodiment, supports a laseroptics assembly and a workpiece having a surface on which a laser beamis incident for laser processing. The multiple stage positioning systemis capable of vibrationally and thermally stable material transport athigh speed and rates of acceleration. A “split axis” design decouplesdriven stage motion along two perpendicular axes lying in separate,parallel planes. In a preferred embodiment, motion in the horizontalplane is split between a specimen (major axis or lower) stage and a scanoptics assembly (minor axis or upper) stage that move orthogonallyrelative to each other.

A dimensionally stable substrate in the form of a granite, or otherstone slab, or a slab of ceramic material, cast iron, or polymercomposite material such as Anocast™, is used as the base for the lowerand upper stages. The slab and the stages are preferably fabricated frommaterials with similar coefficients of thermal expansion to cause thesystem to advantageously react to temperature changes in a coherentfashion. The substrate is precisely cut (“lapped”) such that portions ofits upper and lower stage surfaces are flat and parallel to each other.In a preferred embodiment, a lower guide track assembly that guides alower stage carrying a specimen-holding chuck is coupled to a lowersurface of the substrate. An upper guide track assembly that guides anupper stage carrying a laser beam focal region control subsystem iscoupled to an upper surface of the substrate. Linear motors positionedalong adjacent rails of the guide track assemblies control the movementsof the lower and upper stages.

The massive and structurally stiff substrate isolates and stabilizes themotions of the laser optics assembly and the specimen, absorbsvibrations, and allows for smoother acceleration and decelerationbecause the supporting structure is inherently rigid. The stiffness ofthe substrate and close separation of the stage motion axes result inhigher frequency resonances, and less error in motion along all threeaxes. The substrate also provides thermal stability by acting as a heatsink. Moreover, because it is designed in a compact configuration, thesystem is composed of less material and is, therefore, less susceptibleto expansion when it undergoes heating. An oval slot cut out of themiddle of the substrate exposes the specimen below to the laser beam andallows for vertical motion of the laser optics assembly through thesubstrate. Otherwise, the specimen is shielded by the substrate fromparticles generated by overhead motion, except for the localized regionundergoing laser processing.

A laser beam focal region control subsystem is supported above the lowerstage and includes a vertically adjustable optics assembly positionedwithin a rigid air bearing sleeve mounted to the upper stage by asupport structure. The rigidity of the support structure allows forfaster and more accurate vertical motion along the beam axis. The innersurface of the sleeve acts as an outer race, and the outer surface ofthe lens acts as an inner race, thus forming an air bearing guiding thevertical motion of the focal region of the laser beam. Vertical motionis initiated by a lens forcer residing at the top end of the sleeve,which imparts a motive force to the optics assembly to adjust its heightrelative to the workpiece on the lower chuck, and in so doing, adjuststhe focal region of the laser relative to the work surface. An isolationflexure device, rigid along the beam axis and compliant in thehorizontal plane, buffers excess motion of the lens forcer from theoptics assembly.

The split axis stage design is applicable to many platforms used insemiconductor processing including dicing, component trim, fuseprocessing, inking, printed wire board (PWB) via drilling, routing,inspection, and metrology. The advantages afforded by such a design arealso of benefit to a whole class of mechanical machining tools.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a decoupled, multiple stage positioningsystem.

FIG. 2 is a partly exploded isometric view of the positioning system ofFIG. 1, showing upper and lower stages that, when the system isassembled, are mounted to a dimensionally stable substrate such as astone slab.

FIG. 3 is an isometric view of the positioning system of FIG. 1, showingthe upper stage supporting a scan lens and upper stage drive components.

FIG. 4 is an isometric view of the positioning system of FIG. 1, showingthe lower stage supporting a specimen-holding chuck and lower stagedrive components.

FIGS. 5A, 5B, and 5C are diagrams showing alternative guide trackassembly configurations for moving one or both of the upper and lowerstages of the positioning system of FIGS. 1-4.

FIG. 6 is an exploded view of a preferred embodiment of a laser beamfocal region control subsystem that includes an air bearing sleeveassembly housing a scan lens and guiding its vertical (Z-axis) motion.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a decoupled, multiple stage positioning system 10,which, in a preferred embodiment, supports components of a laserprocessing system through which a laser beam propagates for incidence ona target specimen. Positioning system 10 includes a dimensionally stablesubstrate 12 made of a stone slab, preferably formed of granite, or aslab of ceramic material, cast iron, or polymer composite material suchas Anocast™. Substrate 12 has a first or upper flat major surface 14 anda second or lower flat major surface 16 that has a stepped recess 18.Major surfaces 14 and 16 include surface portions that are planeparallel to each other and conditioned to exhibit flatness andparallelism within about a ten micron tolerance.

A surface portion of upper major surface 14 and a first guide trackassembly 20 are coupled to guide movement of a laser optics assemblystage 22 along a first axis, and a surface portion of lower majorsurface 16 and a second guide track assembly 24 are coupled to guidemovement of a specimen stage 26 along a second axis that is transverseto the first axis. Optics assembly stage 22 supports a laser beam focalregion control subsystem 28, which includes a scan lens 30 that dependsdownwardly below lower major surface 16 of substrate 12. Specimen stage26 supports a specimen-holding chuck 32. The guided motions of stages 22and 26 move scan lens 30 relative to laser beam processing locations ona surface of a specimen (not shown) held by chuck 32.

In a preferred implementation, substrate 12 is set in place so thatmajor surfaces 14 and 16 define spaced-apart horizontal planes and guidetrack assemblies 20 and 24 are positioned so that the first and secondaxes are perpendicular to each other and thereby define respective Y-and X-axes. This split axis architecture decouples motion along the X-and Y-axes, simplifying control of positioning the laser beam and chuck32, with fewer degrees of freedom allowed.

FIG. 3 shows in detail optics assembly stage 22, which operates withfirst guide track assembly 20 shown in FIG. 2. First guide trackassembly 20 includes two spaced-apart guide rails 40 secured to supportportions of upper major surface 14 and two U-shaped guide blocks 42supported on a bottom surface 44 of optics assembly stage 22. Each oneof guide blocks 42 fits over and slides along a corresponding one ofrails 40 in response to an applied motive force. A motor drive foroptics assembly stage 22 includes a linear motor 46 that is mounted onupper major surface 14 and along the length of each guide rail 40.Linear motor 46 imparts the motive force to propel its correspondingguide block 42 for sliding movement along its corresponding guide rail40. Each linear motor 46 includes a U-channel magnet track 48 that holdsspaced-apart linear arrays of multiple magnets 50 arranged along thelength of guide rail 40. A forcer coil assembly 52 positioned betweenthe linear arrays of magnets 50 is connected to bottom surface 44 ofoptics assembly stage 22 and constitutes the movable member of linearmotor 46 that moves optics assembly stage 22. A suitable linear motor 46is a Model MTH480, available from Aerotech, Inc., Pittsburgh, Pa.

Each rail guide 40-guide block 42 pair of first guide track assembly 20shown in FIG. 2 is a rolling element bearing assembly. Alternatives forguide track assembly 20 include a flat air bearing or a vacuum preloadedair bearing. Use of either type of air bearing entails removal of eachguide rail 40, exposing the surface portions of upper surface 14 to formguide surfaces, and substitution for each guide block 42 the guidesurface or bearing face of the bearing, which is attached to bottomsurface 44 of laser optics assembly stage 22. Vacuum preloaded airbearings, which have a pressure port and a vacuum port, hold themselvesdown and lift themselves off the guide surface at the same time. Use ofvacuum preloaded air bearings needs only one flat guide surface; whereasuse of opposed bearing preloading needs two flat, parallel guidesurfaces. Suitable air bearings are available from New Way MachineComponents, Inc., Aston, Pa. Thus, depending on the type of guide trackassembly used, surface portions of upper major surface 14 may representa guide rail mounting contact surface or a bearing face noncontactingguide surface.

A pair of encoder heads 60 secured to bottom surface 44 of opticsassembly stage 22 and positioned adjacent different ones of guide blocks42 includes position sensors that measure yaw angle and distancetraveled of optics assembly stage 22. Placement of the position sensorsin proximity to guide rails 40, guide blocks 42, and linear motors 46driving each of stages 22 and 26 ensures efficient, closed-loop feedbackcontrol with minimal resonance effects. A pair of stop members 62 limitsthe travel distance of guide blocks 42 in response to limit switchesincluded in encoder heads 60 that are tripped by a magnet (not shown)attached to substrate 12. A pair of dashpots 64 dampen and stop themotion of optics assembly stage 22 to prevent it from overtravelmovement off of guide rails 40.

An oval slot 66 formed in substrate 12 between and along the lengths ofguide rails 40 provides an opening within which scan lens 30 can travelas optics assembly stage 22 moves along guide rails 40. A pair ofthrough holes 68 formed in the region of stepped recess 18 in substrate12 provides operator service access from upper surface 14 to encoderheads 60 to maintain their alignment.

FIG. 4 shows in detail specimen stage 26 in operative association withsecond guide track assembly 24 of FIG. 2. Second guide track assembly 24includes guide rails, U-shaped guide blocks, linear motors, U-channelmagnet tracks, magnets, forcer coil assemblies, and encoder heads thatcorrespond to and are identified by the same reference numerals as thosedescribed above in connection with first guide track assembly 20. Linearmotors 46 and the components of and components supported by second guidetrack assembly 24 are mounted on a surface 70 of a specimen stage bed72.

The mechanical arrangement of stages 22 and 26 and motors 46 results inreduced pitch and roll of stages 22 and 26, and enhances accuracy ofhigh velocity motion. Symmetric placement of motors 46 on opposite sidesof stages 22 and 26 improves control of yaw. The placement of motors 46along the sides of stages 22 and 26, as opposed to underneath them,minimizes thermal disturbance of critical components and positionsensors.

Second guide track assembly 24 and specimen stage 26 supporting chuck 32fits into and is secured within stepped recess 18. Surface 70 ofspecimen stage bed 72 is secured against a surface portion 74 of lowermajor surface 16 adjacent the wider, lower portion of stepped recess 18,and chuck 32 is positioned below the innermost portion of stepped recess18 of lower major surface 16 and moves beneath it in response to themotive force imparted by linear motors 46 moving specimen stage 26 alongsecond guide track assembly 24. A pair of stop members 76 limits thetravel distance of guide blocks 42 in response to limit switchesincluded in encoder heads 60 that are tripped by a magnet (not shown)attached to substrate 12. A pair of dashpots 78 dampen and stop themotion of specimen stage 26 to prevent it from overtravel movement offof guide rails 40.

A first alternative to guide track assembly 24 is a magnetic preloadedair bearing using specimen stage bed 72 as a bearing land or guideway.Use of a magnetic preloaded air bearing entails removal of each guiderail 40, exposing the surface portions of specimen stage bed 72, and theremoval of each guide block 42, providing on the bottom surface ofspecimen stage 26 space for mounting the air bearing with its (porous)bearing face positioned opposite the exposed surface portion.

FIG. 5A is a schematic diagram showing the placement of two magneticpreloaded air bearings 100 in the this first alternative arrangement. Asteel plate, or steel laminate structure 102, is fixed on surface 70 inthe space between and along the lengths of forcer coil assemblies 52.Two spaced-apart flat air bearings 100 are fixed to correspondingsurface portions 104 of a bottom surface 106 of specimen stage 26 andrun along the lengths of linear motors 46. A suitable air bearing is asilicon carbide porous media flat bearing series Part No. S1xxxxx,available from New Way Machine Components, Inc., Aston, Pa. A sheetmagnet 108 is positioned in the space between air bearings 100 on bottomsurface 106 of specimen stage 26 and spatially aligned with steel plate102 so that the exposed surfaces of magnet 108 and steel plate 102confront each other. The magnetic force of attraction urges sheet magnet108 downwardly toward steel plate or steel laminate 102 as indicated bythe downward pointing arrow in FIG. 5A, and the net force of airbearings 100 urges specimen stage 26 upwardly away from surface 70 fromspecimen stage bed 72, as indicated by two parallel upward pointingarrows in FIG. 5A. The simultaneous application of opposed magneticforce and pressurized air creates a thin film of air in spaces 110between (porous) bearing faces 112 of air bearings 100 and bearingguideways 114 on surface 70. The lift force of air bearings 100 equalstwice the sum of the weight of specimen stage 26 and the magnetic forceof magnet 108. Linear motors 46 impart the motive force that results innearly zero friction motion of specimen stage 26 along the lengths ofbearing guideways 114.

A second alternative to guide track assembly 24 is a vacuum preloadedair bearing using specimen stage bed 72 as a bearing land or guideway.Similar to the above-described first alternative to guide track assembly20, use of a vacuum preloaded air bearing entails removal of each guiderail 40, exposing surface portion 114 of specimen stage bed 72, and theremoval of each guide block 42, providing on bottom surface 106 ofspecimen stage 26 space for mounting the vacuum loaded air bearing, withits pressure land positioned opposite exposed surface portion 114.

FIG. 5B is a schematic diagram showing the placement of two vacuumpreloaded air bearings 120 in the second alternative arrangement. Twospaced-apart vacuum preloaded air bearings 120 are fixed tocorresponding surface portions 104 of bottom surface 106 of specimenstage 26 and run along the lengths of linear motors 46. A suitable airbearing is a vacuum preloaded air bearing series Part No. S20xxxx,available from New Way Machine Components, Inc., Aston, Pa. Vacuumpreloaded bearings 120 simultaneously hold themselves down and liftthemselves off bearing guideways 114 on surface 70. Each vacuumpreloaded bearing 120 has a pressure land that is divided intospaced-apart land portions 122 a and 122 b. A vacuum area 124 is locatedbetween land portions 122 a and 122 b. The simultaneous application anddistribution of air pressure and vacuum pressure creates a thin film ofair in spaces 126 between pressure land portions 122 a and 122 b ofvacuum preloaded air bearings 120 and bearing guideways 114 on surface70. Linear motors 46 impart the motive force that results in nearly zerofriction motion of specimen stage 26 along the lengths of the bearingguideways 114.

A third alternative to guide track assembly 24 entails the use of eithera magnetic preloaded air bearing of the first alternative, or a vacuumpreloaded air bearing of the second alternative in the absence ofspecimen stage bed 72, as well as each guide rail 40 and each guideblock 42.

FIG. 5C is a schematic diagram showing specimen stage 26 riding onmagnetic preloaded air bearings or vacuum preloaded air bearings 140along bottom surface 142 of substrate 12. When substrate 12 is in ahorizontal disposition, magnetic preloaded or vacuum preloaded airbearings 140 develop sufficient force to overcome the gravitationalforce on specimen stage 26 as it rides along bottom surface 142. Skilledpersons will appreciate that laser optics assembly stage 22 cansimilarly be adapted to ride on magnetic preloaded air bearings orvacuum preloaded air bearings along upper major surface 14 of substrate12. The stage configuration can use mechanical linear guides in place ofthe air bearings described above. Other devices for measuring position,such as interferometers, can be implemented in this positioning systemdesign.

The mass of substrate 12 is sufficient to decouple the mass of opticsassembly stage 22 and the mass of specimen stage 26, including thespecimen mounted on it, so that the guided motion of one of stages 22and 26 contributes a negligible motive force to the other one of them.The masses of stages 22 and 26 moving along the X- and Y-axes are low,and thereby allow high acceleration and high velocity processing andlimit heat generation in linear motors 46. Because the center of mass ofthe laser beam focal region control subsystem 28 is aligned with thecenter of mass of optics assembly stage 22, perturbations in the motionof optics assembly stage 22 are minimized.

Laser optics assembly stage 22 has an opening 200 that receives controlsubsystem 28, which includes an air bearing assembly 202 containing scanlens 30. Control subsystem 28 controls the axial position of a laserbeam focal region formed by scan lens 30 as the laser beam propagatesgenerally along a beam axis 206, which is the optic axis of scan lens30, and through scan lens 30 for incidence on a work surface of a targetspecimen supported on specimen stage 26.

FIG. 6 shows in greater detail the components of control subsystem 28and its mounting on laser optics assembly stage 22. With reference toFIG. 6, control subsystem 28 includes a lens forcer assembly 210 that iscoupled by a yoke assembly 212 to scan lens 30 contained in the interiorof an air bushing 214 of air bearing assembly 202. A suitable airbushing is Part No. S307501, available from New Way Machine Components,Inc., Aston, Pa. Lens forcer assembly 210, which is preferably a voicecoil actuator, imparts by way of yoke assembly 212 a motive force thatmoves scan lens 30 and thereby the focal region of the laser beam toselected positions along beam axis 206.

Voice coil actuator 210 includes a generally cylindrical housing 230 andan annular coil 232 formed of a magnetic core around which copper wireis wound. Cylindrical housing 230 and annular coil 232 are coaxiallyaligned, and annular coil 232 moves axially in and out of housing 230 inresponse to control signals (not shown) applied to voice coil actuator210. A preferred voice coil device 210 is an Actuator No. LA 28-22-006Z, available from BEI Kimco Magnetics, Vista, Calif.

Annular coil 232 extends through a generally circular opening 234 in avoice coil bridge 236 having opposite side members 238 that rest onuprights 240 (FIG. 1) mounted on laser optics assembly stage 22 toprovide support for laser beam focal region control subsystem 28. Voicecoil bridge 236 includes in each of two opposite side projections 242 ahole 244 containing a tubular housing 250 through which passes a rod 252extending from an upper surface 254 of a guiding mount 256. Each rod 252has a free end 258. Guiding mount 256 has on its upper surface 254 anannular pedestal 260 on which annular coil 232 rests. Two stacked,axially aligned linear ball bushings 264 fit in tubular housing 250contained in each hole 244 of side projections 242 of voice coil bridge236. Free ends 258 of rods 252 passing through ball bushings 264 arecapped by rod clamps 266 to provide a hard stop of lower travel limit ofannular coil 232 along beam axis 206.

Housing 230 has a circular opening 270 that is positioned in coaxialalignment with the center of annular coil 232, opening 234 of voice coilbridge 236, and the center of annular pedestal 260 of guiding mount 256.A hollow steel shaft 272 extends through opening 270 of housing 230, anda hexagonal nut 274 connects in axial alignment hollow steel shaft 272and a flexible tubular steel member 276, which is coupled to yokeassembly 212 as further described below. Hexagonal nut 274 is positionedin contact with a lower surface 278 of annular coil 232 to driveflexible steel member 276 along a drive or Z-axis 280 in response to thein-and-out axial movement of annular coil 232. Hollow steel shaft 272passes through the center and along the axis of a coil spring 282, whichis confined between a top surface 284 of housing 230 and a cylindricalspring retainer 286 fixed at a free end 290 of hollow steel shaft 272.Coil spring 282 biases annular coil 232 to a mid-point of its strokealong Z-axis 280 in the absence of a control signal applied to voicecoil actuator 210.

Yoke assembly 212 includes opposed yoke side plates 300 (only one shown)secured at one end 302 to a surface 304 of a yoke ring 306 and at theother end 308 to a multilevel rectangular yoke mount 310. Scan lens 30formed with a cylindrical periphery 312 and having an annular top flange314 fits in yoke assembly 212 so that top flange 314 rests on surface304 of yoke ring 306. Scan lens 30 contained in the interior of airbushing 214 forms the inner race of air bearing assembly 202, and aninner surface 316 of air bushing 214 forms the outer race of air bearingassembly 202. The implementation of air bearing assembly 202 increasesthe rigidity of scan lens 30 in the X-Y plane but allows scan lens 30 tomove along the Z-axis in a very smooth, controlled manner.

Flexible steel member 276 has a free end 320 that fits in a recess 322in an upper surface 324 of yoke mount 310 to move it along Z-axis 280and thereby move scan lens 30 along beam axis 206. An encoder head mount326 holding an encoder 328 and attached to voice coil bridge 236cooperates with an encoder body mount 330 holding an encoder scale andattached to guiding mount 256 to measure, using light diffractionprinciples, the displacement of guiding mount 256 relative to voice coilbridge 236 in response to the movement of annular coil 232. Becauseflexible tubular steel member 276 is attached to annular coil 232, thedisplacement measured represents the position of scan lens 30 along beamaxis 206.

A quarter-waveplate 340 secured in place on a mounting ring 342 ispositioned between a lower surface 344 of rectangular yoke mount 310 andtop flange 314 of scan lens 30. A beam deflection device 346, such as apiezoelectric fast steering mirror, attached to optics assembly stage 22(FIG. 3) is positioned between rectangular yoke mount 310 andquarter-waveplate 340. Fast steering mirror 346 receives an incominglaser beam 348 propagating along beam axis 206 and directs laser beam348 through quarter-waveplate 340 and scan lens 30. Quarter-waveplate340 imparts circular polarization to the incoming linearly polarizedlaser beam, and fast steering mirror 346 directs the circularlypolarized laser beam for incidence on selected locations of the worksurface of a target specimen supported on specimen stage 26. When faststeering mirror 346 is in its neutral position, Z-axis 280, beam axis206, and the propagation path of laser beam 348 are collinear. When faststeering mirror 346 is in operation, the propagation path of laser beam348 is generally aligned with beam axis 206.

Flexible steel member 276 is rigid in the Z-axis direction but iscompliant in the X-Y plane. These properties of flexible steel member276 enable it to function as a buffer, isolating the guiding action ofair bearing assembly 202 containing scan lens 30 from the guiding actionof lens forcer assembly 210 that moves scan lens 30.

Lens forcer assembly 210 and air bearing assembly 202 have centers ofgravity and are positioned along the Z-axis. Voice coil bridge 236 oflens forcer assembly 210 has two depressions 350, the depths and crosssectional areas of which can be sized to achieve the axial alignment ofthe two centers of gravity. Such center of gravity alignment eliminatesmoment arms in control system 28 and thereby helps reduce propensity oflow resonant frequency vibrations present in prior art cantilever beamdesigns.

Several examples of possible types of laser processing systems in whichpositioning system 10 can be installed include semiconductor wafer orother specimen micromachining, dicing, and fuse processing systems. In awafer dicing system, laser beam 348 moves along scribe locations on thewafer surface. In a wafer fuse processing system, a pulsed laser beam348 moves relative to wafer surface locations of fuses to irradiate themsuch that the laser pulses either partly or completely remove fusematerial.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

1. In a laser processing system in which a laser beam propagates along abeam axis and through a lens for incidence on a work surface of a targetspecimen mounted on a support, the lens forming a focal region of thelaser beam and the support operatively connected to a multiple-axispositioning system that moves the laser beam and the target specimenrelative to each other to position the laser beam at selected locationson the work surface, the improvement comprising: an air bearing assemblyincluding an air bushing that contains the lens and guides its movementalong the beam axis in response to a motive force applied to the lens toadjust the focal region of the laser beam relative to the work surface.2. The laser processing system of claim 1, in which the air bushing hasan inner surface, and in which the lens has an outer diameter thatdefines a bearing surface of the lens, the bearing surface of the lensoperating as an inner race of the air bearing and the inner surface ofthe air bushing operating as an outer race of the air bushing.
 3. In alaser processing system in which a laser beam propagates along a beamaxis and through a lens for incidence on a work surface of a targetspecimen mounted on a support, the lens forming a focal region of thelaser beam and the support operatively connected to a multiple-axispositioning system that moves the laser beam and the target specimenrelative to each other to position the laser beam at selected locationson the work surface, the improvement comprising: an air bearing assemblyincluding an air bushing that contains the lens and guides its movementalong the beam axis in response to a motive force applied to the lens;and a lens forcer including a movable member that is guided for movementalong the beam axis and is operatively connected to the lens to impartthe motive force to move the lens along the beam axis and thereby adjustthe focal region of the laser beam relative to the work surface.
 4. Thelaser processing system of claim 3, in which the lens forcer includes avoice coil actuator that controls the movement of the movable member. 5.The laser processing system of claim 3, in which the multiple-axispositioning system and lens forcer define a guide element center ofgravity residing at a first location and the air bearing assemblycontaining the lens has a center of gravity residing in a secondlocation that is substantially the same as the first location.
 6. Thelaser processing system of claim 3, further comprising an isolationflexure device operating as a buffer between the lens forcer and thelens to isolate the guided movement of the movable member of the lensforcer from the guided movement imparted to the lens by the movablemember of the lens forcer.
 7. The laser processing system of claim 6, inwhich the isolation flexure device is rigid along the beam axis and iscompliant in a plane transverse to the beam axis.